Prime Minister Jose Luis Rodriguez Zapatero said four months isn't enough time for Bildu to convince people that they are separate from ETA after four decades of Basque terrorism, El Pais reported.
"[Members of Bildu] have advanced, but they must take additional steps, more decisive and firm, to reject, forget and make a definitive break with violence," Zapatero said.
Zapatero said during a rally by his Socialist party in Durango on Sunday that ETA must end its campaign of violence with no conditions or negotiations.
Bildu was formed earlier this month by former members of the outlawed Batasuna party with two legitimate radical left parties Eusko Alkartasuna and Alternatiba.
The nationalist group will announce the 300 candidates running for positions in the May 22 municipal elections on Monday in the Basque Country and Navarre.
Pello Urizar, Eusko Alkartasuna secretary-general, said all the candidates have complied with the law and have renounced ETA violence.